Heparan sulfate (HS) is a partially characterized family of sulfated glycosaminoglycans of unknown function. These molecules are readily isolated from most tissues, most commonly lung, where they form part of the cell surface as well as being located in the extracellular matrix. Because of its location on the cell surface, structural complexity, large size and negative charge, we and others have proposed that HS may be involved in mediation of cell-cell interactions. Our current research is aimed at the elucidation of the structural differences between HS isolated from various cells and tissues and separable by ion-exchange chromatography and/or electrophoresis. In addition, we will test our proposal that the complexity of HS is generated by a family of sulfotransferases involved in the modification of the carbohydrate backbone of HS. The results of these studies will be used as a basis to test our proposal that HS is involved in the determination of specific cell-cell interactions.